[apparatus for growing stoichiometric lithium niobate and lithium tantalate single crystals and method of growing the same]

ABSTRACT

A method for growing stoichiometric lithium niobate and lithium tantalate single crystals is provided. A crystal growing apparatus that includes a long crucible with a separation member therein is used. A solid feed material is quenched from a molten state, solidified in batches or sintered before charged in the long crucible to obtain substantially stoichiometric solids. The separation member divides the long crucible into a melting zone and a feeding zone located under the melting zone, and it could effectively prevent bubble formation in the growing crystal. The stoichiometry of the axial and radial composition can be well controlled, and the control of the diameter of the crystal body is easily achieved as well.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of a prior application Ser. No.10/064,880, filed Aug. 27, 2002, which claims the priority benefit ofTaiwan application Ser. No. 91103520, filed Feb. 27, 2002.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of growing singlecrystals. More specifically, the present invention relates to a methodof growing substantially stoichiometric lithium niobate (LN) and lithiumtantalate (LT) single crystals.

[0004] 2. Description of the Related Art

[0005] LN and LT crystals have been widely used as a photoelectricmaterial, especially for photo-transmission, audio and visual appliance,double-frequency laser, and optical storage medium. A conventionalmethod of growing LN and LT crystals, called as Czochralski method (Czmethod), is well known in the art and as described hereafter.

[0006]FIG. 1 and FIG. 2 respectively show phase diagrams of a congruentcomposition with lithium and niobium or lithium and tantalium. Here, acomposition with a stoichiometric/non-stoichiometric amount of reactantsis also called a stoichiometric/non-stoichiometric composition. That is,in the congruent composition, the mole ratio of lithium to niobium orlithium to tantalium is not about 50%. Therefore, a crystal bodyobtained by the conventional Cz method usually has a non-stoichiometriccomposition. However, stoichiometric LN and LT crystals are preferablebecause of their superior photoelectric properties. A method ofcontinuously growing a stoichiometric LN and LT is thus needed.

[0007] Recently, a double-crucible method has been proposed by NationalInstitute for Research in Inorganic Materials (NIRIM), Japan. FIG. 3 isa schematic, cross-sectional view of an apparatus used in a conventionaldouble-crucible method.

[0008] In FIG. 3, a melt with a lithium-rich composition for growth ofcrystals is placed in two separate crucibles. A conventional crystalgrowing apparatus 300 includes a chamber 302, an external crucible 304,an inner crucible 306, a heater 308, and a crystal pulling system 310.The crystal growing apparatus 300 is provided with a powder feedingsystem 312 for automatically charging the raw material having thestiochiometric composition. The external crucible 304 is arranged insidethe chamber 302. The inner crucible 306 is arranged in the externalcrucible 304 and has a small opening 314 on the sidewall of the innercrucible 306 near a bottom thereof. The heater 308 surrounds the chamber302, including a bottom and a sidewall thereof. The powder feedingsystem 312 is arranged outside the heater 308 and is provided with atube 318 extending into the chamber 302. An outlet of the tube 318 islocated between the external crucible 304 and the inner crucible 306.The crystal pulling system 310 is located above the inner crucible 306.

[0009] When the crystal growing process is performed, the solid rawmaterial with about 58% Li is melted and entered into the inner crucible306 and the external crucible 304 inside the chamber 302. Then, acrystal seed 320 is placed in the crystal pulling system 310 and dippedin the melt 322 within the inner crucible 306. The crystal seed 320 ispulled up at a constant speed while being rotated to grow a crystal body324. As the crystal body 324 is gradually grown, the powder material 326is added into the melt 328 within the external crucible 304 at a ratethat can compensate the consumed material consumption. Meanwhile, thepowder material 326 added in the melt 328 is continuously melted by theheater 308. The melt 328 flows into the inner crucible 306 through theopening 314 of the inner crucible 306. Thereby, the composition of thecrystal body 324 can be constant.

[0010] The double-crucible method grows stoichiometric lithium niobate(LiNbO₃, LN) and lithium tantalate (LiTaO₃, LT) single crystals bykeeping the composition of the melt 422 in the inner crucible 306 atpoint A of the phase diagram of FIG. 1 and FIG. 2. Theoretically, thestoichiometric crystals can be obtained as long as the charged amount ofpowder stuff 326 is precisely controlled to grow the crystal body 324.Automatically feeding powder stuff is requisite for the double-cruciblemethod to grow the stoichiometric crystals. However, it needs high leveltechnology and high production cost. Further, in consideration ofthermal aspect, there is a significant difference between the meltingpoints of the melt 322 and 328 respectively in the inner crucible 306and the external crucible 304, which causes the control of the heater308 difficult. Therefore, the double-crucible method is difficult to becommercialized at a large scale.

SUMMARY OF INVENTION

[0011] It is an object of the present invention to provide a method ofgrowing stoichiometric lithium niobate and lithium tantalate singlecrystals and an apparatus therefor. With the method of the invention, itis easier to obtain the stoichiometric single crystals.

[0012] It is another object of the present invention to provide a methodof growing stoichiometric lithium niobate and lithium tantalate singlecrystals and an apparatus therefor, in which the uniformity of thecomposition of the single crystal can be improved.

[0013] It is still another object of the present invention to provide amethod of growing stoichiometric lithium niobate and lithium tantalatesingle crystals and an apparatus therefor, in which a diameter of thecrystal can be easily controlled.

[0014] It is still another object of the present invention to provide amethod of growing stoichiometric lithium niobate and lithium tantalatesingle crystals and an apparatus therefor, in which power consumption isreduced and thus the production cost is decreased.

[0015] In order to achieve the above and other objectives of theinvention, a crystal growing apparatus of growing stoichiometric lithiumniobate and lithium tantalate single crystals is provided. The crystalgrowing apparatus includes a chamber, a long crucible, a heating system,a separation member, a pushing/rotating system and a crystal pullingsystem. The long crucible is provided with a separation member a meltingzone and a feeding to define a.feeding zone and a melting zone. Theprovision of the separation member further prevents bubbles generatedwhen the solid feed material is melted from being included in thecrystal body. The solid feed material can be quenched from a moltenstate, solidified in batches or sintered to obtain stoichiometric solidsbefore charged into the feeding zone of the long crucible. With thedesign of the melting zone in the apparatus of the present invention,the axial composition and radial composition can be controlled well.Thereby, substantially stoichiometric lithium niobate and lithiumtantalate single crystals can be obtained.

[0016] Furthermore, a method of growing stoichiometric lithium niobateand lithium tantalate single crystals using a crystal growing apparatusis also provided. The crystal growing apparatus includes a chamber, along crucible, a heating system, a pushing/rotating system and a crystalpulling system. In the method of the claimed invention, a solid feedmaterial charged into a lower portion of the long crucible is kept in asolid state. A zone stuff is charged into the long crucible above thesolid feed material. The solid feed material can be quenched from amolten state, solidified in batches or sintered before charged into thelong crucible in order to grow substantially stoichiometric lithiumniobate and lithium tantalate single crystals.

[0017] In another aspect of the invention, a method of growingstoichiometric lithium niobate and lithium tantalate single crystals byusing at least a long crucible and an external heater is provided. Asolid feed material is charged into the long crucible. A separationmember is placed into the long crucible. A solid feed material is placedinto the long crucible on the separation member, and then graduallymelted by the external heater. The long crucible is pushed upward as acrystal body is grown. The zone stuff is separated from the solid feedmaterial by the separation member to further control an admix rate ofsolid feed material/zone stuff. The provision of the separation memberfurther prevents bubbles generated when the solid feed material ismelted from being included in a crystal body. The ratio of the pullingrate of the crystal body and the pushing rate of the long crucible is inproportion to the ratio of the sinner cross section area of the longcrucible and the cross section area of the crystal body in order toobtain the crystal body with uniform composition. Preferably, the ratioof the pulling rate of the crystal body to the pushing rate of the longcrucible is substantially equal to the ratio of the inner cross sectionarea of the long crucible to the cross section area of the crystal body.

[0018] In still another aspect of the invention, an apparatus forgrowing stoichiometric lithium niobate and lithium tantalate singlecrystals is provided. The apparatus includes a chamber, a long crucible,a separation member or an inner crucible, a heating system, apushing/rotating system, and a crystal pulling system. The long crucibleis arranged inside the chamber. The separation member, such as aninsulative plate, a shallow crucible, or a crucible with perforatedwall, is arranged in the long crucible to divide the long crucible intoa melting zone and a feeding zone. The heating system surrounds asidewall of the chamber, corresponding to locations of the melting zoneand the feeding zone. Furthermore, the crystal pulling system is locatedabove the chamber to pull up a crystal seed during crystal growth. Thepushing/rotating system is located under the long crucible to rotate andpush the long crucible up.

[0019] In still another aspect of the invention, an apparatus forgrowing stoichiometric lithium niobate and lithium tantalate singlecrystals is provided. The apparatus includes a chamber, a long crucible,a heating system, a pushing/rotating system, and a crystal pullingsystem. The long crucible is arranged inside the chamber. The heatingsystem surrounds a sidewall of the chamber for melting solids therein.Furthermore, the crystal pulling system is located above the chamber forholding a crystal seed and pulling up the crystal seed during crystalgrowth. The pushing/rotating system is located under the long crucibleto rotate and push the long crucible up.

[0020] In view of foregoing, with the use of the crystal growingapparatus of the present invention, in which the long crucible isprovided with or not provided with a separation member, it is mucheasier to grow a crystal body with a controlled composition.

[0021] Furthermore, in the case of the crystal growing apparatusprovided with the separation member, such as an insulative plate, ashallow crucible or a shallow crucible with a perforated wall isprovided, the zone stuff in the melting zone is separated from the solidfeed material in the feeding zone. The provision of the separationmember further prevents bubbles generated when the solid feed materialis melted from being included in the crystal body. If the separationmember is a crucible with a shallow perforated wall, it is preferable tofix the separation member by three external Pt/Rh rods that are securedat an upper part of the long crucible. It is easy to take the separationmember out of the long crucible after the crystal growth.

[0022] The solid feed material can be quenched from a molten state,solidified in batches or sintered before be charged the long crucible.Therefore, the composition of the grown crystal is more uniform.

[0023] In the apparatus of the present invention, the melting zone ofthe long crucible is located above the feeding zone of the longcrucible. The solid feed material and the zone stuff are separatelyprepared and sequentially charged into the long crucible. Thereby, thestoichiometry of the axial and radial composition can be wellcontrolled, and the control of the diameter of the crystal body iseasily achieved as well.

[0024] Furthermore, the heat required for the present invention isapplied around the sidewall of the external crucible, instead of thewhole apparatus. Therefore, the energy can be saved and the productioncost can be thus reduced.

BRIEF DESCRIPTION OF DRAWINGS

[0025] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

[0026] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciple of the invention.

[0027]FIG. 1 is a phase diagram of LiNbO₃.

[0028]FIG. 2 is a phase diagram of LiTaO₃.

[0029]FIG. 3 is a schematic, cross-sectional view of an apparatus usedin a conventional double-crucible method.

[0030]FIG. 4 is a schematic, cross-sectional view of an apparatus forgrowing stoichiometric lithium niobate and lithium tantalate singlecrystals according to one preferred embodiment of the present invention.

[0031]FIG. 5 is a schematic, cross-sectional view of an apparatus forgrowing stoichiometric lithium niobate and lithium tantalate singlecrystals according to another preferred embodiment of the presentinvention, wherein an example of a separation member is shown.

[0032]FIG. 6 is a flow chart showing a method of growing stoichiometriclithium niobate and lithium tantalate single crystals according to onepreferred embodiment of the present invention.

DETAILED DESCRIPTION

[0033] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Whenever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

[0034] In the invention, a zone melting Czochralski method (hereafter,zone melting Cz method) is used. Stoichiometric lithium niobate (LN) andlithium tantalate (LT) single crystals are obtained by the zone meltingCz method that uses a crystal growing apparatus provided with a longcrucible. The long crucible can be preferably provided with a separationmember therein.

[0035]FIG. 4 is an apparatus for growing stoichiometric lithium niobateand lithium tantalate single crystals according to one preferredembodiment of the present invention. The crystal growing apparatus 400includes a chamber 402, a long crucible 404, a separation member 406, aheating system 408, a pushing/rotating system 410 and a crystal pullingsystem 412. The long crucible 404 is arranged inside the chamber 402.The heating system 408 preferably surrounds sidewall of the chamber 402.The long crucible 404 particularly has a high wall. Preferably, theheight of the wall of the long crucible 404 is larger than its diameter.The separation member 406 is arranged inside the long crucible 404 todefine a melting zone 414 in an upper portion of the long crucible 404and form a feeding zone 416 in a lower portion of the long crucible 404.The provision of the separation member 406 further prevents bubblesgenerated when the solid feed material is melted from being included inthe crystal body. The separation member 406 can be an insulative plate,a shallow crucible, or a crucible with a perforated wall, for example.The separation member 406 can be made of, for example, platinum oriridium. The heating system 408 is arranged outside the chamber 402,corresponding to locations of the feeding zone 416 and the melting zone414. The heating system 408 includes a preheater 418 for preheating thesolid feed material in the feeding zone 416, and a post-heater 420 forgradually reducing a temperature of the grown crystal in the meltingzone 414. The crystal pulling system 412 is arranged above the meltingzone 414 for holding the crystal seed 422 and pulling up the crystalseed 422 during crystal growth. The pushing/rotating system 410 islocated under the long crucible 404 to rotate and push up the longcrucible 404 during crystal growth.

[0036] During crystal growth, the feeding zone 416 in the long crucible404 is charged with a solid feed material. The solid feed material canbe quenched from a molten state, solidified in batches or sinteredbefore charged into the feeding zone 416 to obtain stoichiometricsolids. Alternatively, a solid feed material prepared previously ischarged into the long crucible. The separation member 406 is placed onthe feeding zone 416 of the long crucible 404. Then, a zone stuff havingcomposition at point A of FIG. 1 is placed into the long crucible 404.

[0037] The separation member 406 used here can be an insulative plate, ashallow crucible (as shown in FIG. 4), or a crucible with a perforatedwall (as shown in FIG. 5). The separation member 406 is provided andarranged over the feeding zone 416, then an admix rate of zonestuff/solid feed materila can be further controlled. If the separationmember is a crucible with a shallow perforated wall, it is preferable tofix the separation member by three external Pt/Rh rods that are securedat an upper part of the long crucible. It is easy to take the separationmember out of the long crucible after the crystal growth.

[0038] The separation member 406 is optionally provided. If theseparation member 406 is not provided, a lower portion of the longcrucible 404, where the solid feed material is located, is also calledfeeding zone 416. Similarly, an upper portion of the long crucible 404,where the zone stuff is located, is also called melting zone 414, asillustrated in the above embodiment shown in FIG. 4. The solid feedmaterial in the feeding zone 416 is kept dense so that the solid feedmaterial would not be affected by the temperature of the melting zone414, and the formation of bubbles is minimized. This purpose can beachieved by densification or solidification of the solid feed materialin the feeding zone 416. The condition of densification includes longersintering time or the use of solid feed material with fine particles.The solidification is performed step-by-step so as to reduce thesolidification rate or quenching from a molten state.

[0039] A crystal seed 422 is placed in the crystal pulling system 412and dipped into the zone stuff of the melting zone 414. The seed 422 ispulled up and rotated at a constant speed to grow a crystal body 424,under the condition that the pulling speed Uc of the seed 424 (crosssectional area, Ac) and the upward pushing rate (Uf) of the longcrucible 404 (inner cross sectional area, Af) satisfy the relationship#cUc ×Ac =#f Uf ×Af; #c and #f are the density of the crystal and thefeed, respectively. As the crystal body 424 is gradually grown, thepushing/rotating system 410 pushes the long crucible 404 upward to meltthe solid feed material near the melting zone 414 at a propertemperature. A solid-liquid phase 426 can be thereby kept at asubstantially the same level. The proper temperature recited above canbe the point A of FIG. 1 and FIG. 2, for example. The crystal body has awell-controlled diameter and composition after a self-stabilizationstage of heat transfer. The method of the present invention isparticularly more convenient than that of the prior art. The crystalbody obtained by the present invention has axial and radial compositionswith improved uniformity. More particularly, the stoichiometric ratio ofLithium and Niobium, or Lithium and tantalum is about 1, which means theoptical properties of LN and LT are superior to those obtainednon-stoichiometrically. If a doping system is further used, the solidfeed material in the feeding zone 416 is doped with a dopant which has aconcentration C₀. The zone stuff in the melting zone 414 has a dopantconcentration C₀/K (wherein K is a segregation constant). The dopant canbe magnesium oxide, zinc oxide, manganese, cerium, terbium, or iron.

EXAMPLE

[0040] The following examples illustrate the present invention in moredetail with reference to FIG. 6.

[0041]FIG. 6 is a flow chart of a method of growing stoichiometriclithium niobate and lithium tantalate single crystals according to onepreferred embodiment of the present invention.

[0042] With reference to FIG. 6, the solid feed material is charged inthe long crucible (step 500). The solid feed material has substantiallystoichiometric composition of LN or LT. Preferably, the solid feedmaterial includes at the molar ratio of 1:1. The feed material is eithersolidified in batches, quenching from a molten state, or sintered toobtain stoichiometric solids before charged in the long crucible.Alternatively, the specific powder material which is previously preparedis charged in the long crucible.

[0043] The separation member 406 is placed in the long crucible (step502). A zone stuff is prepared (step 504). The zone stuff includes58%-60% Li₂O/(Li₂O+Nb₂O₅) or 58-60% Li₂O/(Li₂O+Ta₂O₅). The zone stuff ischarged into the long crucible (step 506). The zone stuff is melted(step 508). A molten phase of the solid feed material that near themelting zone is also separated from a non-molten phase of the solid feedmaterial by the separation member. The non-molten phase of the solidfeed material in the feeding zone 416 is kept dense. Therefore, the step502 is preferable because it helps not only to prevent bubbles generatedwhen the solid feed material is melted from being included in thecrystal body, but also to well control the admix rate of the solid feedmaterial in the feeding zone and the zone stuff in the melting zone.

[0044] The crystal seed 422 is dipped in the zone stuff in the meltingzone by means of a crystal pulling system (step 510). After the crystalseed is melted, the crystal pulling system 412 pulls the crystal seed upwhile rotating the crystal seed to grow a crystal body (step 512). Thezone stuff is consumed as the crystal is grown, and replenished with theunderlying feed material that has turn into melt near the melting zone414. Therefore, the long crucible 404 is pushed upward by means of, forexample, a pushing/rotating system as the crystal body is grown (step514).

[0045] Further, the pushing rate of the long crucible 404 is in relationwith the pulling rate of the crystal body. That is, the ratio of thepulling rate of the crystal body and the pushing rate of the longcrucible, depending on the density of the feed, is about equal to theratio of the inner cross sectional area of the long crucible and thecross sectional area of the crystal body. For example, when the crystalbody to be grown has a diameter that is half of the diameter of the longcrucible, the pushing rate of the long crucible is one fourth of thepulling rate of the crystal body. In a step 516, once the crystal bodywith a desired length is obtained, the crystal body is pulled out of theliquid level of the zone stuff melt at a higher pulling speed, and thenis cooled down slowly. The crystal body is removed after cooling to roomtemperature.

[0046] The present invention has the following advantages over the priorart.

[0047] 1. With the apparatus of the present invention having the longcrucible with or without the separation member, it is much easier togrow a crystal body with a controlled composition.

[0048] 2. The solid feed material can be solidified in batches, quenchedfrom a molten state, or sintered before charged into the long crucible.The zone stuff is consumed as the crystal is grown, and replenished withthe underlying feed material that has turn into melt near the meltingzone.

[0049] 3. By defining the melting zone in the long crucible, thestoichiometry and the dopant concentration of the axial and radialcomposition can be well controlled. Furthermore, the control of thediameter of the crystal body is easily achieved.

[0050] 4. The required heat is applied around the sidewall of theexternal crucible, instead of the whole apparatus. Therefore, energyconsumption and thus fabrication cost are reduced.

[0051] 5. The arrangement of the separation member in the long cruciblefurther enables the control the admix rate of the zone stuff in themelting zone and the solid feed material in the feeding zone. It alsoprevents bubbles generated by melting the solid feed material from beingincluded in the crystal body.

[0052] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the forgoing, it is intended that the present invention covermodifications and variations of this invention provided they fall withinthe scope of the following claims and their equivalents.

1. An apparatus for growing stoichiometric lithium niobate and lithium tantalate single crystals, the apparatus comprising: a chamber, a crystal pulling system, a long crucible arranged inside the chamber; a separation member arranged in the long crucible to divide an inner space of the long crucible into a melting zone and a feeding zone, wherein the melting zone is located above the feeding zone; a heating system surrounding a sidewall of the chamber; and a pushing/rotating system arranged under the long crucible to rotate and push the long crucible.
 2. The apparatus of claim 1, wherein the separation member comprises an insulative plate, a shallow crucible, and a crucible with a perforated wall.
 3. The apparatus of claim 1, wherein the separation member is made of a material selected from platinum.
 4. The apparatus of claim 1, wherein the separation member is made of a material selected from iridium.
 5. The apparatus of claim 1, further comprising a preheater outside the chamber, corresponding to the position of the feeding zone.
 6. The apparatus of claim 1, further comprising a post-heater outside the chamber, corresponding to the upper portion of the long crucible.
 18. An apparatus for growing stoichiometric lithium niobate and lithium tantalate single crystals, the apparatus comprising: a chamber, a crystal pulling system arranged above the chamber, a long crucible arranged inside the chamber; a heating system surrounding sidewall of the chamber; and a pushing/rotating system arranged under the long crucible to rotate and push up the long crucible.
 19. The apparatus of claim 18, wherein the heating system further comprises a preheater outside the chamber, corresponding to the position of the feeding zone.
 20. The apparatus of claim 18, wherein the heating system further comprises a post-heater outside the chamber corresponding to the upper portion of the long crucible. 